Distribution of status information from several virtual output queus over a plurality of switch cards of a packet switching device

ABSTRACT

The invention relates to a packet switching device comprising at least one line card ( 1  and  2 ) with a port control ( 6  and  7 ) for storage and arrangement of packets in output queues and for generating status information of the output queue, and at least one switch card ( 3  to  5 ) consisting of a crosspoint matrix ( 8 ) and an arbiter ( 9 ) to control the crosspoint matrix ( 8 ), the port control ( 6  and  7 ) being provided for distributing the status information over several arbiters ( 9 ). Each responding arbiter ( 9 ) is provided to cyclically receive a respective part of the status information from the port control.

[0001] The invention relates to a packet switching device comprising aplurality of switch cards.

[0002] The publication “Weighted Arbitration Algorithms with Prioritiesfor Input Queued Switches with 100% Throughput” by R. Schoenen, G. Post,G. Sander, Broadband Switching Symposium '99, compares various weightedswitching algorithms of a packet switching device. The switchingalgorithms attempt with various switching steps to prevent a collisionof several packets intended for the same output port of the packetswitching device and reduce the resulting data loss or delay. Use ofstatus information on the output queues of data packets is notconsidered.

[0003] It is an object of the invention to guarantee collision-freeswitching of data in the form of packets.

[0004] The object is achieved by a packet switching device of the typedescribed in the opening paragraph in that

[0005] at least one line card is provided with a port control forstorage and arrangement of packets in output queues and for generatingstatus information of the output queue, and

[0006] at least one switch card is provided consisting of a crosspointmatrix and an arbiter to control the crosspoint matrix,

[0007] where the port control is provided to distribute the statusinformation of several output queues over several arbiters and

[0008] the arbiter of the switch card is provided to cyclically receivea respective part of the status information.

[0009] A packet switching device comprises several line cards andseveral switch cards. The line card includes as an essential component aport control. Each port control is connected with a plurality ofparallel switch cards and its task is to arrange the incoming packets tothe packet switching device by priority and desired output port of thepacket switching device. The arrangement of the packets by priority anddesired output port of the packet switching device in the port controlis known as the virtual output queue (VOQ). The port control hasinformation on the condition, known as status information, of thevirtual output queue and sends this to the switch cards at regularintervals.

[0010] Each switch card consists of a crosspoint matrix and an arbiter.The configuration of the crosspoint matrix is redetermined at regularintervals by the arbiter and new links produced between the input andoutput ports of the packet switching device for switching the packets.

[0011] To calculate each link, the port control sends arbiter statusinformation so that calculation can be optimized as a function of thestatus information.

[0012] The port control can send the new status information distributedvia the arbiters. Each arbiter receives part of the status informationfrom the port control. By receiving only part of the status informationa maximum required bandwidth can be kept constant for exchange ofinformation between the port control and an arbiter. The constantbandwidth between the port control and an arbiter is independent of thenumber of arbiters connected with the port control.

[0013] When distributing the status information the port control doesnot distinguish between the individual switch cards. Thus the number ofswitch cards has no effect on the functionality of the port control.Also the absence of one or more switch cards during operation has noeffect on the function of the port control. The port control need notperform any additional functions if the packet switching device issupplemented with an additional switch card during operation.

[0014] The port control of a line card is connected at the same timewith several parallel arbiters of the switch cards.

[0015] To ensure that in the event of a fault on a switch card, thefunction of the packet switching device is not disrupted too, the portcontrol works independently of the number of switch cards provided arequired switching capacity is maintained. By varying the number ofswitch cards, a throughput and delay of the switched packets can beinfluenced. By addition and removal, a scaling of the power of thepacket switching device can be achieved without additional devices. Aswitch card can be operated both individually in a packet switchingdevice and also—without additional changes in its function, speed,interface etc.—as one of many in a packet switching device.

[0016] If after a fault or addition the switch card is reclassified inoperation, the arbiter of this switch card has no status information onthe output queues. Nonetheless the arbiter calculates a configuration ofits relevant crosspoint matrix and sends this to the port control andthe crosspoint matrix. Each port control of the packet switching devicereceives the calculated (default) configuration and replies with thecurrent (updated) status information. On the next calculation thearbiter takes into account the status information previously received.

[0017] Depending on the number of current changes of the statusinformation the arbiter has therefore received, after the m-thcalculation, most status information available, where m is equalapproximately to the number of input and output ports of the packetswitching device. The more redundant status information is sent from theport control to the arbiter, the faster the arbiter can work suitablyagain.

[0018] As each port control is simultaneously connected with severalswitch cards, it requires an interface which can be operatedsimultaneously with several switch cards. The internal operating speedof the port control is equal to n times the operating speed of a switchcard, where n is equal to the number of line cards of the packetswitching device.

[0019] An increase of operation speed by multiple use of the switchcards causes no change to the switch card or the speed of the linksbetween the port control and the switch card. The switch card and thespeed of the connections fulfil the requirements of a packet switchingdevice with just one switch card, known as the minimal system.

[0020] A decision of the port control to use a particular switch card istriggered by the arbiter. The arbiter of each switch card at regularintervals transmits its calculated default configuration of thecrosspoint matrix to the port control even if the calculation is basedon incorrect and incomplete status information. The port controlresponds to the information from the arbiter. The port control does notdistinguish each individual arbiter but considers all arbiters togetheras one arbiter and orientates itself to different frequencies with whicha configuration was sent, and the physical connections to the arbiter.

[0021] By means of weighting, the port control can inform the arbiterwhether and how urgently the packets in the input port should beswitched. Weighting can be sent within the status information to thearbiter and consists of a function of all status information for anoutput port. Weighting can contain data on the priority and class of thepackets or the waiting time and size of an output queue.

[0022] As the arbiter receives a copy of the weighting value receivedmost recently, the receipt of a difference between the current value ofthe weighting and the value received most recently is sufficient. Onecondition is a complete value received most recently, which is absent inthe case of a fault on the arbiter. For this reason a complete valuerepresenting all current status information is sent.

[0023] The invention also relates to a line card with a port control forstorage and arrangement of packets in queues and to generate statusinformation on the queues for a packet switching device with at leastone switch card comprising a crosspoint matrix and an arbiter to controlthe crosspoint matrix. The port control distributes the statusinformation to several arbiters, where each answering arbiter isdesigned for cyclic reception in each case of part of the statusinformation from the port control.

[0024] The invention also relates to a switch card consisting of acrosspoint matrix and an arbiter to control the crosspoint matrix for apacket switching device with at least one line card with a port controlfor storage and arrangement of packets in output queues, and to generatestatus information of the queues. The port control distributes thestatus information to several arbiters where each answering arbiter isprovided for cyclic reception in each case of part of the statusinformation from the port control.

[0025] The invention will be further described with reference to anexample of embodiment shown in the drawings, to which, however, theinvention is not restricted. These show:

[0026]FIG. 1 a packet switching device and

[0027]FIG. 2 administration of the status information within the portcontrol.

[0028] A packet switching device shown in FIG. 1 consists of two linecards 1 and 2 and several switch cards 3 to 5. The line cards 1 and 2each consist of different components not relevant for the description ofthe example of embodiment and therefore not shown, such as an opticaltransfer unit, frame generator, network processor etc., and a portcontrol 6 and 7 essential for the example of embodiment. The number (n)of port controls is often much greater than two, in the example ofembodiment it is restricted to two for clarity. Each switch card 3 to 5consists of a crosspoint matrix 8 and an arbiter 9. Both the arbiter 9and the crosspoint matrix 8 of each parallel switch card 3 to 5 areconnected with the respective port control 6 and 7.

[0029] Data packets of constant length are known as cells. As cells ofconstant length are easier to process in switching than packets ofvarying size, the incoming packets are distributed within the line cards1 and 2 into cells of constant length and stored temporarily in outputqueues. After successful switching i.e. accepted allocation of an inputport with an output port, the cells are removed from the output queue.

[0030] The task of the port control 1 and 2 is to arrange the incomingcells in output queues by priority and desired output port of the packetswitching devices. The arrangement is known as a virtual output queue(VOQ). Each port control 1 and 2 determines the status information ofits respective output queue and transmits the status information to thearbiter 9 at regular intervals.

[0031] The arbiter 9 for each switch line card 3 to 5 calculates atregular intervals known as cell periods an optimum configuration of thecrosspoint matrix 8 for linking the input and output ports of the packetswitching device. The calculations of the arbiters 9 are based on thestatus information from the port controls 6 and 7 in which the cells arewaiting in output queues for switching to an output port. The links mustfulfil two conditions. Within a cell period a link cannot connectseveral output ports with one input port and/or several input ports withone output port of the packet switching device. The arbiter 9 containsall current status information received from the port control 6 and 7and in each cell period sends the calculated configuration of thecrosspoint matrix 8 to the port control 6 and 7 and the crosspointmatrix 8.

[0032] As switch cards 3 to 5 are operated in parallel, an operatingspeed of each switch card 3 to 5 is less than the internal operatingspeed of the port controls 6 and 7. Thus the cell period of the switchcards 3 to 5 is longer than an internal cell period of the port control1 and 2. As in each case the port control 1 and 2 is connectedsimultaneously with several switch cards 3 to 5, it requires aninterface which can be operated simultaneously with several switch cards3 to 5. The internal operating speed of port controls 6 and 7 is threetimes the operating speed of the switch card 3 to 5.

[0033] After receiving the configuration calculated by the arbiter 9 andhence the links, the port control knows to which switch card 3 to 5 itis connected in the next cell period.

[0034]FIG. 2 shows the administration of status information within theport controls 6 and 7. The port controls 6 and 7 each contain a table 10with status information 11 to 19. Weighting generators 20 to 24 with adisplay 25 to 29 are also required to generate the weighting andindicate a change in weighting. A round robin 1 method 30 and a roundrobin 2 method 31 are used as scheduling strategies for access controlat an output 34 to the arbiter 9.

[0035] In the table 10 are established all status information 11 to 19and changes in status information 11 to 19 since the last dispatch tothe arbiter. The weighting generator 20 to 24 generates the weightingwhich, using the round robin 1 method 30 as a scheduling strategy,receives access to an arbiter 9 through the output 34. In generating theweighting, only the last changed status information is taken intoaccount. If no changed status information is present since the last cellperiod, the weighting determined by the round robin 2 method 31 is sent.In each cell period two weightings are simultaneously sent to thearbiter 9.

[0036] The higher the number of switch cards 3 to 5, the larger theamount of status information that each port control 6 and 7 sends to thearbiter 9. In order to keep the available bandwidth between the portcontrol 6 and 7 and switch card 3 to 5 constant, each port control 6 and7 distributes only part of the status information modified last to thearbiters 9 of the switch cards 3 to 5.

1. A packet switching device comprising at least one line card (1 and 2)with a port control (6 and 7) for storage and arrangement of packets inoutput queues and for generating status information of the output queue,and at least one switch card (3 to 5) consisting of a crosspoint matrix(8) and an arbiter (9) to control the crosspoint matrix (8), where theport control (6 and 7) is provided to distribute the status informationover several arbiters (9) and each responding arbiter (9) is provided tocyclically receive a respective part of the status information from theport control.
 2. A packet switching device as claimed in claim 1,characterized in that at least one line card (1 and 2) of the packetswitching device is connected with several parallel switch cards (3 to5).
 3. A packet switching device as claimed in claim 1, characterized inthat by removal and addition of the switch cards (3 to 5), a scaling ofthe power of the packet switching device is provided.
 4. A packetswitching device as claimed in claim 1, characterized in that aplurality of redundant switch cards (3 to 5) are provided as replacementfor faulty switch cards (3 to 5).
 5. A packet switching device asclaimed in claim 1, characterized in that an internal operating speed ofthe port control (6 and 7) has n times the value of an operating speedof a switch card (3 to 5).
 6. A packet switching device as claimed inclaim 1, characterized in that the arbiter (9) of each switch card (3 to5) is provided to transmit at regular intervals to the crosspoint matrix(8) and the port control (6 and 7) a calculated configuration of thecrosspoint matrix (8).
 7. A packet switching device as claimed in claim1, characterized in that a weighting is provided consisting of afunction of all status information for an output port of the packetswitching device, to notify the arbiter (9).
 8. A line card (1 and 2)comprising a port control (6 and 7) for storage and arrangement ofpackets in output queues and to generate status information of theoutput queues for a packet switching device comprising at least oneswitch card (3 to 5) consisting of a crosspoint matrix (8) and anarbiter (9) to control the crosspoint matrix (8), where the port control(6 and 7) is provided to distribute the status information to severalarbiters (9), and each responding arbiter (9) is provided to cyclicallyreceive a respective part of the status information from the portcontrol.
 9. A switch card comprising a crosspoint matrix (8) and anarbiter (9) to control the crosspoint matrix (8) for a packet switchingdevice comprising at least one line card (1 and 2) with a port control(6 and 7) for storage and arrangement of packets in queues and togenerate status information of the queue, where the port control (6 and7) is provided to distribute the status information to a plurality ofarbiters (9) and each answering arbiter (9) is provided to cyclicallyreceive a respective part of the status information from the portcontrol.